Contention groups for hidden nodes

ABSTRACT

Communicating among stations in a network includes, from each of multiple stations in the network, transmitting information indicating which other stations from which that station is able to reliably receive transmissions. A schedule for communicating among the stations is determined based on the information from the stations and transmitting the schedule over the network. The schedule includes a plurality of time slots during which respective sets of stations are assigned to communicate using a contention-based protocol.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 60/941,949, entitled “MANAGING COMMUNICATIONS OVER A SHARED MEDIUM,”filed on Jun. 4, 2007, which is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to managing contention groups for hidden nodes.

BACKGROUND

A network of communication stations can share a communication medium(e.g., wires connecting multiple stations or spectrum for transmittingradio signals among stations) using any of a variety of accesstechniques. Some access techniques (e.g., carrier sense multiple access(CSMA) techniques) include a contention period in which stations contendfor use of the medium for transmitting a signal by sensing when themedium is idle. In CSMA techniques, “collisions” sometimes occur whensignals from two or more stations overlap. Some CSMA techniques attemptto detect collisions and abort transmission to reduce the negativeimpact of collisions (e.g., CSMA/CD techniques). Other CSMA techniquesinclude mechanisms to avoid or reduce the probability of collisions(e.g., CSMA/CA techniques). For example, different transmissions may beassigned one of multiple priorities. Access is granted using a PriorityResolution Period in which stations signal the priority at which theyintend to transmit, and only the highest priority transmissions areallowed to continue in the contention process. A random backoffmechanism spreads the time over which stations attempt to transmit,thereby reducing the probability of collision.

SUMMARY

In one aspect, in general, a method for communicating among stations ina network includes, from each of multiple stations in the network,transmitting information indicating which other stations from which thatstation is able to reliably receive transmissions. The method includesdetermining a schedule for communicating among the stations based on theinformation from the stations and transmitting the schedule over thenetwork. The schedule includes a plurality of time slots during whichrespective sets of stations are assigned to communicate using acontention-based protocol.

Aspects can include one or more of the following features.

Each station in a given set is able to reliably receive transmissionsfrom all the other stations in the given set.

The contention-based protocol does not use Request to Send/Clear To Send(RTS/CTS) transmissions among the stations in a given set.

At least some of the sets include one or more stations that are not ableto reliably receive transmissions from at least some other stations inthe same set.

The contention-based protocol used by at least some of the sets thatinclude one or more stations that are not able to reliably receivetransmissions from at least some other stations in the same set useRequest to Send/Clear To Send (RTS/CTS) transmissions among the stationsin a given set.

At least some of the stations are not able to reliably receivetransmissions from at least some of the other stations in the network.

Each station in a given set is able to reliably receive transmissionsfrom all the other stations in the given set.

Each station in a first set of stations assigned to communicate in agiven time slot is not able to reliably receive transmissions from anyof the stations in a second set of stations assigned to communicate inthe given time slot.

The contention-based protocol does not use Request to Send/Clear To Send(RTS/CTS) transmissions among the stations in a given set.

At least some of the sets include one or more stations that are not ableto reliably receive transmissions from at least some other stations inthe same set.

The contention-based protocol used by at least some of the sets thatinclude one or more stations that are not able to reliably receivetransmissions from at least some other stations in the same set useRequest to Send/Clear To Send (RTS/CTS) transmissions among the stationsin a given set.

The information indicating which other stations from which a station isable to reliably receive transmissions is transmitted to a firststation.

The first station determines the schedule.

The first station assigns stations to sets based on the information.

The method further comprises providing the stations in different setsone or more parameters associated with the contention-based protocolindependently.

The one or more parameters include at least one of a contention windowsize and a defer counter.

The one or more parameters provided to a given set include at least oneparameter that is selected based on the number of stations in the givenset.

The one or more parameters provided to a given set include a randombackoff parameter.

The contention-based protocol comprises a carrier sense multiple access(CSMA) protocol.

The CSMA protocol comprises a carrier sense multiple access withcollision avoidance (CSMA/CA) protocol.

In another aspect, in general, a method for communicating among stationsin a network includes assigning a first station to a first level;assigning stations not assigned to a preceding level that can reliablyreceive transmissions from a preceding level to a higher level;assigning stations to sets based on the assigned level; and determininga schedule for communicating among the stations and transmitting theschedule over the network. The schedule includes a plurality of timeslots during which respective sets of stations are assigned tocommunicate using a contention-based protocol.

Aspects can include one or more of the following features.

At least some of the stations are not able to reliably receivetransmissions from at least some of the other stations in the network.

Each station in a given set is able to reliably receive transmissionsfrom all the other stations in the given set.

Each station in a first set of stations assigned to communicate in agiven time slot is not able to reliably receive transmissions from anyof the stations in a second set of stations assigned to communicate inthe given time slot.

The contention-based protocol does not use Request to Send/Clear To Send(RTS/CTS) transmissions among the stations in a given set.

At least some of the sets include one or more stations that are not ableto reliably receive transmissions from at least some other stations inthe same set.

The contention-based protocol used by at least some of the sets thatinclude one or more stations that are not able to reliably receivetransmissions from at least some other stations in the same set useRequest to Send/Clear To Send (RTS/CTS) transmissions among the stationsin a given set.

In another aspect, in general, a system for communicating among stationsin a network includes a first set of multiple stations assigned tocommunicate during a first time slot using a contention-based protocol;a second set of multiple stations assigned to communicate during asecond time slot using a contention-based protocol; and a third set ofmultiple stations assigned to communicate during the first time slotusing a contention-based protocol. At least one station in the first setis able to reliably communicate with at least one station in the secondset, and none of the stations in the first set is able to reliablycommunicate with any of the stations in the third set.

Aspects can include one or more of the following features.

Each of multiple stations in the network is configured to transmitinformation indicating which other stations from which that station isable to reliably receive transmissions.

At least one station in the network is configured to determine aschedule for communicating among the stations based on the informationfrom the stations and to transmit the schedule over the network.

The schedule includes the first and second time slots.

Among the many advantages of the invention (some of which may beachieved only in some of its various aspects and implementations) arethe following.

The approaches described can improve network efficiency. For example,inefficiencies due to hidden nodes can be reduced by reducing oreliminating hidden nodes, and by trading off the number of contentiongroups and the reduction in hidden nodes. Contention groups can beassigned to time slots such that network communication resources can bereused efficiently. Beacon transmissions can be used to simplifyassignment of contention groups and facilitate distributed protocols forforming the groups.

Other aspects and advantages will be apparent from the detaileddescription, drawings, appendices and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a communication network.

FIG. 2 is a block diagram of a communication system for communicatingover the network.

FIGS. 3A and 3B are timing diagrams of a TDMA scheduling period.

FIGS. 4A-4C and FIG. 6 are schematic diagrams of stations assigned tocontention groups and time slot groups.

FIG. 5 is a diagram of stations grouped into beacon levels.

DETAILED DESCRIPTION

There are a great many possible implementations of the invention, toomany to describe herein. Some possible implementations that arepresently preferred are described below. It cannot be emphasized toostrongly, however, that these are descriptions of implementations of theinvention, and not descriptions of the invention, which is not limitedto the detailed implementations described in this section but isdescribed in broader terms in the claims.

FIG. 1 shows an exemplary network configuration for an access network100 such as a Broadband Power Line Network (BPLN) that provides accessto a backhaul network. A BPLN can be managed by a service providerentity having access to the underlying physical power line medium. ABPLN is a general purpose network that can be used for several types ofapplications including, smart grid management, broadband internetaccess, voice and video delivery services, etc. In variousimplementations, a BPLN can be deployed on low voltage, medium voltageand high voltage power lines. Additionally, a BPLN can span an entireneighborhood or it may be deployed within a single multi-dwelling unit.For example, it can be used to provide network service to tenants in asingle apartment building. While power lines are one medium fordeploying the BPLN, similar techniques can be deployed on other wirelines, such as, for example, coaxial cables, twisted pair or acombination thereof.

A BPLN can include one or more Cells. A cell is a group of broadbandpower line (BPL) devices in a BPLN that have similar characteristicssuch as association management, security, Quality of Service (QoS) andchannel access settings, for example. Cells in a BPLN are logicallyisolated from each other, and communication to and from the backhauloccurs within the cell. Each cell in a BPLN includes a Core-Cell and mayalso include one or more Sub-Cells. There can be more than one cell on agiven physical power line medium.

A Core-Cell includes a group of devices in a BPLN that can share certainfunctionality such as a common security protocol. An exemplary Core-Cellincludes a Head End (HE), Repeaters (R), and Network Termination Units(NTUs), but may exclude Customer Premise Equipment (CPE). The Head End(HE) is a device that bridges a cell to the backhaul network. At a giventime, a cell will have one active Head End and the Head End manages thecell including the Core-Cell and any associated Sub-Cells. A Repeater(RP) is a device that selectively retransmits MSDUs to extend theeffective range and bandwidth of the BPLN Cell. Repeaters can alsoperform routing and QoS functions. The NTU is a device that connects aBPLN cell to the end users' network or devices. The NTU may in somecases bridge to other network technologies such as WiFi. A single NTUcan serve more than one customer. Each Sub-Cell is associated with anactive NTU. In some implementations, an HE, an NTU and/or an RP can beco-located at a single station. Thus, a single device may be designed toperform multiple functions. For example, a single device cansimultaneously be programmed to perform the tasks associated with an RPand an NTU.

Various types of CPE devices (e.g., a computer) can be used as endpointnodes in the network and such devices can communicate with other nodesin the network through the NTU, any number of repeaters, (e.g.,including no repeaters), and the Head End. Each node in the networkcommunicates as a communication “station” using a PHY layer protocolthat is used by the nodes to send transmissions to any other stationsthat are close enough to successfully receive the transmissions.Stations that cannot directly communicate with each other use one ormore repeater stations to communicate with each other. The stations havethe potential to interfere with each other, but techniques can be usedto coordinate in a centralized and/or distributed manner.

Any of a variety of communication system architectures can be used toimplement the portion of the network interface module that converts datato and from a signal waveform that is transmitted over the communicationmedium. An application running on a station provides and receives datato and from the network interface module in segments. A “MAC ServiceData Unit” (MSDU) is a segment of information received by the MAC layer.The MAC layer can process the received MSDUs and prepares them togenerate “MAC protocol data units” (MPDUs). An MPDU is a segment ofinformation including a header (e.g., with management and overheadinformation) and payload fields that the MAC layer has asked the PHYlayer to transport. An MPDU can have any of a variety of formats basedon the type of data being transmitted. A “PHY Protocol Data Unit (PPDU)”refers to the modulated signal waveform representing an MPDU that istransmitted over the power line by the physical layer.

Apart from generating MPDUs from MSDUs, the MAC layer can provideseveral functions including channel access control, providing therequired QoS for the MSDUs, retransmission of corrupt information,routing and repeating. Channel access control enables stations to sharethe powerline medium. Several types of channel access control mechanismslike carrier sense multiple access with collision avoidance (CSMA/CA),centralized Time Division Multiple Access (TDMA), distributed TDMA,token based channel access, etc., can be used by the MAC. Similarly, avariety of retransmission mechanism can also be used. The Physical layer(PHY) can also use a variety of techniques to enable reliable andefficient transmission over the transmission medium (power line, coax,twisted pair etc). Various modulation techniques like OrthogonalFrequency Division Multiplexing (OFDM), Wavelet modulations can be used.Forward error correction (FEC) code like Viterbi codes, Reed-Solomoncodes, concatenated code, turbo codes, low density parity check code,etc., can be employed by the PHY to overcome errors.

One implementation of the PHY layers is to use OFDM modulation. In OFDMmodulation, data are transmitted in the form of OFDM “symbols.” Eachsymbol has a predetermined time duration or symbol time T_(s). Eachsymbol is generated from a superposition of N sinusoidal carrierwaveforms that are orthogonal to each other and form the OFDM carriers.Each carrier has a peak frequency f_(i) and a phase Φ_(i) measured fromthe beginning of the symbol. For each of these mutually orthogonalcarriers, a whole number of periods of the sinusoidal waveform iscontained within the symbol time T_(s). Equivalently, each carrierfrequency is an integral multiple of a frequency interval Δ_(f)=1/T_(s).The phases Φ_(i) and amplitudes A_(i) of the carrier waveforms can beindependently selected (according to an appropriate modulation scheme)without affecting the orthogonality of the resulting modulatedwaveforms. The carriers occupy a frequency range between frequencies f₁and f_(N) referred to as the OFDM bandwidth.

Referring to FIG. 2, a communication system 200 includes a transmitter202 for transmitting a signal (e.g., a sequence of OFDM symbols) over acommunication medium 204 to a receiver 206. The transmitter 202 andreceiver 206 can both be incorporated into a network interface module ateach station. The communication medium 204 can represent a path from onedevice to another over the power line network.

At the transmitter 202, modules implementing the PHY layer receive anMPDU from the MAC layer. The MPDU is sent to an encoder module 220 toperform processing such as scrambling, error correction coding andinterleaving.

The encoded data is fed into a mapping module 222 that takes groups ofdata bits (e.g., 1, 2, 3, 4, 6, 8, or 10 bits), depending on theconstellation used for the current symbol (e.g., a BPSK, QPSK, 8-QAM,16-QAM constellation), and maps the data value represented by those bitsonto the corresponding amplitudes of in-phase (I) and quadrature-phase(Q) components of a carrier waveform of the current symbol. This resultsin each data value being associated with a corresponding complex numberC_(i)=A_(i) exp(jΦ_(i)) whose real part corresponds to the I componentand whose imaginary part corresponds to the Q component of a carrierwith peak frequency f_(i). Alternatively, any appropriate modulationscheme that associates data values to modulated carrier waveforms can beused.

The mapping module 222 also determines which of the carrier frequenciesf₁, . . . , f_(N) within the OFDM bandwidth are used by the system 200to transmit information. For example, some carriers that areexperiencing fades can be avoided, and no information is transmitted onthose carriers. Instead, the mapping module 222 uses coherent BPSKmodulated with a binary value from the Pseudo Noise (PN) sequence forthat carrier. For some carriers (e.g., a carrier i=10) that correspondto restricted bands (e.g., an amateur radio band) on a medium 204 thatmay radiate power no energy is transmitted on those carriers (e.g.,A₁₀=0). The mapping module 222 also determines the type of modulation tobe used on each of the carriers (or “tones”) according to a “tone map.”The tone map can be a default tone map, or a customized tone mapdetermined by the receiving station, as described in more detail below.

An inverse discrete Fourier transform (IDFT) module 224 performs themodulation of the resulting set of N complex numbers (some of which maybe zero for unused carriers) determined by the mapping module 222 onto Northogonal carrier waveforms having peak frequencies f₁, . . . , f_(N).The modulated carriers are combined by IDFT module 224 to form adiscrete time symbol waveform S(n) (for a sampling rate f_(R)), whichcan be written as

$\begin{matrix}{{S(n)} = {\sum\limits_{i = 1}^{N}{A_{i}{\exp\left\lbrack {j\left( {{2\;\pi\;{\mathbb{i}}\;{n/N}} + \Phi_{i}} \right)} \right\rbrack}}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where the time index n goes from 1 to N, Ai is the amplitude and Φ_(i)is the phase of the carrier with peak frequency f_(i)=(i/N)f_(R), andj=√−1. In some implementations, the discrete Fourier transformcorresponds to a fast Fourier transform (FFT) in which N is a power of2.

A post-processing module 226 combines a sequence of consecutive(potentially overlapping) symbols into a “symbol set” that can betransmitted as a continuous block over the communication medium 204. Thepost-processing module 226 prepends a preamble to the symbol set thatcan be used for automatic gain control (AGC) and symbol timingsynchronization. To mitigate intersymbol and intercarrier interference(e.g., due to imperfections in the system 200 and/or the communicationmedium 204) the post-processing module 226 can extend each symbol with acyclic prefix that is a copy of the last part of the symbol. Thepost-processing module 226 can also perform other functions such asapplying a pulse shaping window to subsets of symbols within the symbolset (e.g., using a raised cosine window or other type of pulse shapingwindow) and overlapping the symbol subsets.

An Analog Front End (AFE) module 228 couples an analog signal containinga continuous-time (e.g., low-pass filtered) version of the symbol set tothe communication medium 204. The effect of the transmission of thecontinuous-time version of the waveform S(t) over the communicationmedium 204 can be represented by convolution with a function g(τ;t)representing an impulse response of transmission over the communicationmedium. The communication medium 204 may add noise n(t), which may berandom noise and/or narrowband noise emitted by a jammer.

At the receiver 206, modules implementing the PHY layer receive a signalfrom the communication medium 204 and generate an MPDU for the MAClayer. An AFE module 230 operates in conjunction with an Automatic GainControl (AGC) module 232 and a time synchronization module 234 toprovide sampled signal data and timing information to a discrete Fouriertransform (DFT) module 236.

After removing the cyclic prefix, the receiver 206 feeds the sampleddiscrete-time symbols into DFT module 236 to extract the sequence of Ncomplex numbers representing the encoded data values (by performing anN-point DFT). Demodulator/Decoder module 238 maps the complex numbersonto the corresponding bit sequences and performs the appropriatedecoding of the bits (including de-interleaving and descrambling).

Any of the modules of the communication system 200 including modules inthe transmitter 202 or receiver 206 can be implemented in hardware,software, or a combination of hardware and software.

Various stations in a network may generate regular beacon transmissionsfor various purposes. A beacon transmission (or simply a “beacon”)includes management information that can be used for a variety ofpurposes. The stations may communicate with each other in time periodsbetween beacon transmissions, provided the power line channelcharacteristics between any two communicating stations permit it.

In some networks one of the functions of a beacon transmission is tocarry medium allocation (or scheduling) information. The schedulinginformation allocates some of the time between beacon transmissions as acontention period during which stations may contend for access to thepower line medium. The scheduling information also allocates acontention-free period during which times slots are assigned toparticular stations for access to the power line medium. The schedulinginformation is provided relative to a TDMA Scheduling Period Start Time(or TDMA Period Start Time).

The TDMA Period start time is synchronized with respect to the AC linecycle of the power line waveform such that the time between consecutiveTDMA period start times is based on the underlying AC line cyclefrequency. Thus, to the extent that the AC line cycle frequency mayvary, the TDMA period start time (and hence the duration of each TDMAPeriod) may not be perfectly periodic. Since the beacons transmitted bythe HE may not be heard by every station, each station transmits its ownbeacon to relay the information in the beacon to stations that do nothear the HE. While the stations may transmit their beacons at differenttimes, the TDMA period start time is established with respect to theinformation contained in the beacons transmitted by the HE. At eachstation, the TDMA period start time can be synchronized to that of theHE using a start time synchronization procedure described in more detailbelow. Each station can then predict a given TDMA period end time (orfuture TDMA period start time) based on the synchronized start time andthe local AC line cycle at the given station by correlating thesynchronized start time to a detectable feature of the power linewaveform such as a zero crossing. The TDMA period can be set by the HEto any multiple of a half of the AC line cycle period, for example, bywaiting for a given number of zero crossings.

In some cases it is desirable to increase the TDMA period to make moreefficient use of the medium by reducing the percentage of time devotedto sending the “overhead” information in the beacon transmission. Thereis also overhead information associated with transmissions from thestations during each TDMA period. It may also be desirable to keep theTDMA period small enough to provide a desired number of transmissionopportunities in a given length of time to reduce the latency. Thus, theTDMA period can be selected according to a trade-off between keepingoverhead low and latency between transmission opportunities low. Forexample, in some implementations the TDMA period is selected to be twicethe AC line cycle period. In this case, when operating in power lineenvironments with an AC line cycle frequency of 60 Hz, the TDMA periodwould be approximately 33.33 msec. When operating in power lineenvironments with an AC line cycle frequency of 50 Hz, the TDMA periodwould be approximately 40 msec. Variations in the TDMA period may occurdue to drift in the AC line cycle frequency. The HE determines theduration of the TDMA period as well as the start time of the TDMAperiod.

FIG. 3A shows the structure of an exemplary TDMA period 300 whichconsists of a Contention Period 302 followed by a Stay-out Period 304and a Contention Free Period 306. In general, a TDMA period can containany number of Contention Periods, Stay-out Periods and Contention FreePeriods in any order. The TDMA period may also be different fordifferent stations in the BPLN. The Contention Period 302 is a time inwhich stations can contend for permission to transmit using a sharedmedium access protocol such as CSMA/CA. The Stay-out Period 304 is atime during which stations are not allowed to transmit. The ContentionFree Period 306 includes time slots assigned for use by predeterminedstations (e.g., using a Time Domain Multiple Access (TDMA) protocol).Beacons are typically transmitted during the Contention Period. Beaconmay also be transmitted during Contention Free Periods. The frequency ofbeacon transmission depends on how frequently the associated managementinformation needs to be communicated. Typically, beacons are transmittedonce in several TDMA periods.

In networks in which repeaters are used to extend the reach of signalsin the network, multiple stations can retransmit beacon transmissions tobe heard by stations that would otherwise not be able to receive thebeacon transmissions from the HE.

In some implementations, contention occurs over all the stations in thenetwork contending during the contention period 302. However, for largenetworks, even with the use of random backoff intervals, collisions aremore likely to occur (e.g., due to multiple layers of hidden nodes). Insome implementations, the stations can be broken into groups thatperform contention independently (e.g., in different contention periods)without interfering with each other. Each contention group has a smallernumber of stations, and can be selected to reduce or eliminate hiddennodes within each group.

For example, the stations can be assigned to “contention groups”centrally by the HE station based on information collected from thestations about which stations can reliably receive transmissions fromwhich other stations, or in a distributed manner as described in moredetail below. Within each contention group, a contention-based protocol(e.g., a CSMA protocol) can be used. Different (potentially overlapping)time slots can be allocated (e.g., as determined by the HE) in whichcontention-based communication occurs based on knowledge of trafficconditions and of routing and network topology. These contention groupscan reduce inefficiencies associated with using a contention-basedprotocol over the entire network that result, for example, from havingmultiple layers of hidden stations. If hidden stations exist in a groupof contending stations, to avoid collisions, Request To Send/Clear ToSend (RTS/CTS) signals can be used. However, these RTS/CTS haveassociated overhead (e.g., time used to send the signals) that reduceefficiency. In some implementations, stations are assigned to contentiongroups so that each contention group does not have any hidden stations(i.e., such that all the stations in a given contention group can heareach other). Without hidden stations among the stations in a contentiongroup, the need for using RTS/CTS signals can be eliminated, improvingefficiency for each group. In some implementations, as described in moredetail below, stations are assigned to contention groups to reduce thelayers of hidden stations, while still allowing some hidden nodes toexists in order to reduce the number of contention groups.

Different contention groups can also be assigned different time slots sothat stations in neighboring contention groups do not interfere. Forexample, the time slots can be assigned during the Contention FreePeriod 306 to ensure that contention occurs between stations assigned tothe same contention group without interference from stations assigned todifferent contention groups. One or more contention groups that do notinterfere (e.g., if the contention groups are far enough apart) and areassigned to the same time slot are called a “time slot group.” Time slotgroups facilitate channel reuse, as explained in more detail below. Thecontention groups also enable customizing parameters to each contentiongroup based on characteristics of the group, such as the number ofstations in the group. Different backoff parameters, for example, can beused. Other parameters that can be customized for each contention groupare a contention window size and a defer counter.

FIG. 4A shows an example, of an assignment of contention groups and timeslot groups in a case in which there is a linear chain of stationsincluding an HE station and repeater stations R1, R2, R3, R4, etc. InFIG. 4A, stations that can hear each other are connected with a line.Thus, HE can hear R1, R1 can hear HE, and R2, and so on. Each station isassigned a position in the chain such that the HE station is in position0, repeater station R1 is in position 1, repeater station R2 is inposition 3, and so on. In this example, in order to eliminate hiddennodes within contention groups and ensure that no contention groupsinterfere with each other, only two time slots are needed. The stationsare divided into contention groups of two and each contention group isassigned to one of two time slot groups (Group A and Group B), startingwith HE and R1 in time slot Group A, R2 and R3 in time slot Group B, andR4 and R5 in time slot Group A, and so on (alternating assigningadjacent contention groups to different time slot groups). All stationswith a position equal to 4n or 4n+1 (where n=0, 1, 2, . . . ) in timeslot Group A contend during a first time slot, and all stations with aposition equal 4n+2 or 4n+3 (where n=0, 1, 2, . . . ) in time slot GroupB contend during a second time slot that does not overlap with the firsttime slot. The start and end times of the time slots for each time slotgroup can be indicated in a message sent by the HE station and passed onto all stations in the network. FIG. 3B shows the time slot 316A used asa contention period for time slot group A and time slot 316B used as acontention period for time slot group B.

In some network topologies, complete elimination of hidden nodes canresult in large number of contention groups. Since more contentiongroups generally call for more time slots for contention, this reducesthe network efficiency. Thus, there is a trade off between the selectionof a larger number of contention groups without hidden nodes and asmaller number of contention groups with hidden nodes. In the lattercase, a smaller number of contention groups will result in lowerefficiency loss due to grouping, however, the need to handle hiddennodes will result in efficiency loss due to collisions and use ofRTS/CTS.

FIG. 4B and FIG. 4C show different groupings of contention groups andassignment to time slot groups for the same network topology. Contentiongroups are shown with dashed ovals and time slot groups are labeled. InFIG. 4B, the grouping is such that there are no hidden nodes in any ofthe contention groups. Three separate time slots (for three respectivetime slot groups) are required to schedule all of these groups so thattransmissions by stations in one group do not interfere with thetransmissions by stations in another group. FIG. 4C shows anothergrouping in which only two time slots (for two respective time slotgroups) are required to schedule them. However, the single contentiongroup assigned to time slot group B has hidden nodes, and the collisionsand the RTS/CTS mechanisms used to address this hidden node problem willresult in some efficiency loss. The grouping used in FIG. 4C providesbetter network efficiency than the grouping used in FIG. 4B. Thegrouping used in FIG. 4C requires only two time slots, which results inmore channel reuse than that used in FIG. 4B.

In some implementations, the HE station can gather information aboutwhich stations can hear which other stations and management messagesfrom the HE tell the stations which group they are assigned to. In someimplementations, the stations can perform a distributed protocol, todetermine which stations can hear each other and select contentiongroups such that the stations in a given group can hear each other.

Each station in the BPLN broadcasts a beacon transmission regularly butnot necessarily at the same time (or at the same frequency) at which theHE broadcasts beacon transmissions. For example, a Repeater station maytransmit its beacon within a given target maximum delay (e.g., 100 ms)after reception of the beacon it is tracking. The beacon transmissioncan contain many fields including a Beacon Level (BL) field. The BL isused to determine the hierarchy of stations in the network. The HE isconsidered to be a Beacon Level 0 station and it sets the BL field inthe beacon transmission to be zero. All the stations that can reliablyreceive (or “hear”) the beacon transmissions sent by the HE (with BL=0)are Beacon Level 1 stations and set the BL field in their beacontransmissions to 1. For all other stations, the BL is one higher thanthe smallest BL of all the stations it can reliably hear. FIG. 5 showsan example of stations grouped into Beacon Levels. A group of stationslabeled HE and S1-S6 may be a head end and a group of repeaters in aBPLN, for example. The stations are shown arranged in a graph with aline between stations that represents the ability to reliably receive abeacon transmission. In this example, stations S1 and S2 are BeaconLevel 1 stations and S3, S4, S5 and S6 are Beacon Level 2 stations.

Beacon Level information can be used by stations to determine thecontention groups and time slot groups in a distributed manner. Forexample, contention groups include stations of two consecutive beaconlevels, and stations whose beacon levels are {4n, 4n+1} (where n=0, 1,2, . . . ) belong to one time slot group and stations whose beaconlevels are {4n+2,4n+3} (where n=0, 1, 2, . . . ) will belong to a secondtime slot group. This grouping will result in a schedule with two timeslots. Another grouping could result in requiring three time slots withstations whose beacon levels are {6n,6n+1} (where n=0, 1, 2, . . . )belong to a time slot group scheduled in the first time slot, those withbeacon levels {6n+2,6n+3}(where n=0, 1, 2, . . . ) belong to a time slotgroup scheduled in the second time slot and, those with beacon levels{6n+4,6n+5}(where n=0, 1, 2, . . . ) belong to a time slot groupscheduled in the third time slot. FIG. 6 shows a network with the beaconlevel of each station shown next to it. Using the beacon level groupingmechanism and considering a schedule of two time slots, stations withbeacon levels 0, 1, 4, 5 belong to time slot Group 1 and stations withbeacon levels 2, 3 belong to time slot Group 2. Hence stations HE, R1,R5, R6, R7 and R8 belong to time slot Group 1 and stations R2 and R3belong to time slot Group 2.

Many other implementations of the invention other than those describedabove are within the invention, which is defined by the followingclaims.

What is claimed is:
 1. A method for communicating among stations in anetwork, the method comprising: receiving, at a first station in thenetwork from remaining respective stations in the network, informationindicating which other stations from which that respective station iscapable of reliably receiving transmissions; determining, at the firststation, a schedule for communicating among the stations based on theinformation from the remaining respective stations and transmitting theschedule over the network; and assigning the stations to contentiongroups based on respective station levels, wherein at least onecontention group includes stations from two different, consecutivelevels; wherein, in the at least one contention group, the first stationis at a first level, wherein each other station is at one level higherthan the lowest level of stations from which it is capable of reliablyreceiving transmissions; wherein the schedule includes a plurality oftime slots, wherein respective contention groups are each assigned toone of the plurality of time slots, wherein stations in the samecontention group contend with each other to communicate over the networkusing a contention-based protocol.
 2. The method of claim 1, whereineach station in a given contention group is capable of reliablyreceiving transmissions from all other stations in the given contentiongroup.
 3. The method of claim 2, wherein the contention-based protocoldoes not use Request to Send/Clear To Send (RTS/CTS) transmissions amongstations in the given contention group.
 4. The method of claim 1,wherein at least some of the contention groups include one or morestations that are not capable of reliably receiving transmissions fromat least some other stations in the same contention group.
 5. The methodof claim 4, wherein for said at least some of the contention groups thatinclude one or more stations that are not capable of reliably receivingtransmissions from at least some other stations in the same contentiongroup, the contention-based protocol includes Request to Send/Clear ToSend (RTS/CTS) transmissions among stations in a given contention group.6. The method of claim 1, wherein at least some of the stations are notcapable of reliably receiving transmissions from at least some otherstations in the network.
 7. The method of claim 6, wherein each stationin a given contention group is capable of reliably receivingtransmissions from all other stations in the given contention group. 8.The method of claim 7, wherein each station in a first contention groupassigned to communicate in a given time slot is not capable of reliablyreceiving transmissions from any stations in a second contention groupassigned to communicate in the same given time slot.
 9. The method ofclaim 7, wherein the contention-based protocol does not include Requestto Send/Clear To Send (RTS/CTS) transmissions among stations in a givencontention group.
 10. The method of claim 6, wherein at least some ofthe contention groups include one or more stations that are not capableof reliably receiving transmissions from at least some other stations inthe same contention group.
 11. The method of claim 10, wherein thecontention-based protocol used by at least some of the contention groupsthat include one or more stations that are not capable of reliablyreceiving transmissions from at least some other stations in the samecontention group includes Request to Send/Clear To Send (RTS/CTS)transmissions among stations in a given contention group.
 12. The methodof claim 1, wherein the first station assigns stations to contentiongroups based on the information.
 13. The method of claim 1, furthercomprising providing the stations in different contention groups one ormore parameters associated with the contention-based protocolindependently.
 14. The method of claim 13, wherein the one or moreparameters include at least one of a contention window size and a defercounter.
 15. The method of claim 13, wherein the one or more parametersprovided to a given contention group include at least one parameter thatis selected based on a number of stations in the given contention group.16. The method of claim 15, wherein the one or more parameters providedto a given contention group include a random backoff parameter.
 17. Themethod of claim 1, wherein the contention-based protocol comprises acarrier sense multiple access (CSMA) protocol.
 18. The method of claim17, wherein the CSMA protocol comprises a carrier sense multiple accesswith collision avoidance (CSMA/CA) protocol.
 19. The method of claim 1,wherein the information indicating which other stations from which thatrespective station is capable of reliably receiving transmissions isdetermined based on repeated beacon transmissions sent from theindicated other stations and received by that respective station. 20.The method of claim 19, wherein beacon transmissions from a givenstation identify one of the respective station levels associated withthe given station.
 21. The method of claim 1, wherein the respectivestation levels form a hierarchy of stations in the network in which eachstation in a set at a first level of the hierarchy is capable ofreliably receiving transmissions from at least one station in a set at asecond level of the hierarchy.
 22. The method of claim 21, wherein thehierarchy includes at least three levels and no stations in a set at athird level of the hierarchy are capable of reliably receivingtransmissions from any station in the set at the second level.
 23. Themethod of claim 22, wherein each station in the set at the third levelis capable of reliably receiving transmissions from at least one stationin the set at the first level.
 24. A method for communicating amongstations in a network, the method comprising: assigning stations tocontention groups based on respective station levels, wherein at leastone contention group includes stations from two different, consecutivelevels; wherein, in the at least one contention group, a first stationis at a first level, wherein each other station is at one level higherthan the lowest level of stations from which it is capable of reliablyreceiving transmissions; and determining a schedule for communicatingamong the stations and transmitting the schedule over the network,wherein said determining the schedule is performed at least in partbased on the contention groups; wherein the schedule includes aplurality of time slots, wherein respective contention groups are eachassigned to one of the plurality of time slots, wherein stations in thesame contention group contend with each other to communicate over thenetwork using a contention-based protocol.
 25. The method of claim 24,wherein at least some of the stations are not capable of reliablyreceiving transmissions from at least some other stations in thenetwork.
 26. The method of claim 25, wherein each station in a givencontention group is capable of reliably receiving transmissions from allother stations in the given contention group.
 27. The method of claim26, wherein each station in a first contention group assigned tocommunicate in a given time slot is not capable of reliably receivingtransmissions from any stations in a second contention group assigned tocommunicate in the given time slot.
 28. The method of claim 26, whereinthe contention-based protocol does not use Request to Send/Clear To Send(RTS/CTS) transmissions among stations in the given contention group.29. The method of claim 25, wherein at least some of the contentiongroups include one or more stations that are not capable of reliablyreceiving transmissions from at least some other stations in the samecontention group.
 30. The method of claim 29, wherein for said at leastsome of the contention groups that include one or more stations that arenot capable of reliably receiving transmissions from at least some otherstations in the same contention group, the contention-based protocolincludes Request to Send/Clear To Send (RTS/CTS) transmissions amongstations in a given contention group.
 31. A system for communicatingamong stations in a network, the system comprising: a first set ofmultiple stations assigned to a first contention group, wherein stationsin the first contention group contend with each other to communicateover a network using a contention-based protocol; a second set ofmultiple stations assigned to a second contention group, whereinstations in the second contention group contend with each other tocommunicate over the network using a contention-based protocol; and athird set of multiple stations assigned to a third contention group,wherein stations in the third contention group contend with each otherto communicate over the network slot using a contention-based protocol;wherein at least one contention group of the first, second, and thirdcontention groups includes stations from two different, consecutivelevels; wherein, in the at least one contention group, a first stationis at a first level, wherein each other station is at one level higherthan the lowest level of stations from which it is capable of reliablyreceiving transmissions; wherein each of multiple stations in thenetwork is configured to transmit information indicating which otherstations from which that station is able to reliably receivetransmissions, at least one station in the first set is capable ofreliably communicating with at least one station in the second set, andnone of the stations in the first set is capable of reliablycommunicating with any of the stations in the third set, and wherein atleast one station in the network is configured to determine a schedulefor each contention group to communicate using the network and totransmit the schedule over the network, the schedule includes at least afirst time slot and a second time slot, wherein stations in the firstcontention group are scheduled in the first time slot, stations in thesecond contention group are scheduled in the second time slot, andstations in the third contention group are scheduled in the first timeslot.